/ Dictionary / Index O opacus: Meaning and Definition of Find definitions for: o•pa•cus Pronunciation: (ō-pā'kus), [key] — adj. Meteorol. (of a cloud) dense enough to obscure the sun or moon. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. opacity opah Related Content Daily Word Quiz: panacea Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language